The invention relates to a temperature-responsive linkage element, and to temperature-responsive linkages operating with such element. Especially in fire protection systems, there are applications where a conduit comprises a valve body which is pushed by a force in the opening or closing sense, but which is kept in the open (in the case of a ventilation tube), or the closed position (in the case of an automatic sprinkler of a possible seat of a fire) because the valve body is retained by a linkage. The latter is, however, designed to give way as soon as the ambient temperature reaches an abnormal level, and the protection system then enters into action, under the influence of the force.
In general, such a linkage connects two bodies (or two parts of the same deformable body) by means of a temperature-responsive linkage element which gives way when it reaches the critical temperature. The presently used linkage elements are, e.g., heat-disintegratable bodies, e.g., by fusion or weakening of which the mechanical resistance strongly decreases or disappears when the critical temperature is reached. In this sense, small sealed bulbs of fragile glass are used. The bulbs are filled with a liquid that evaporates at a critical temperature, so that the receptacle is broken by the expansion of the liquid. Linkage systems in which the releasing movement is procured by a bimetallic strip are also known.
The existing linkage elements, however, experience difficulties in achieving all the necessary or desirable requirements for such elements. The requirement of short response time, essential for fire protection systems, seriously limits the dimension of the heat-sensitive body. This limits the order of magnitude of the forces that the linkage is capable of withstanding and the releasing force that the element is capable of producing. This is contrary to the requirement that these forces should be as high as possible. The use of bimetallic strips presents the difficulty of a rather large necessary strip volume to produce sufficient releasing force, hence a high response time, and such a strip works progressively without having a sharply defined reaction temperature. Fuse alloys show a better defined reaction temperature but, as they stand under normal conditions rather near to the weakening point, the forces acting on the linkage cause the body to flow slowly. Evaporation glass bulbs have a sharply defined reaction temperature, but the response time is high due to the liquid that must be heated, and they are fragile.